1. Field of the Invention
The present invention relates to the treatment of hot gases from either combustion, gasification or pyrolysis processes. More specifically, the invention relates to a granular bed filtration apparatus for conditioning the above said gases.
2. Description of the Prior Art
Classical power plants and incinerators produce electrical power by generating steam with hot combustion gases in a boiler and using the steam to drive steam turbines. Advanced high efficiency power generation methods are based on gasification or pyrolysis of appropriate feedstocks and use of the resulting synthetic gas either in burners coupled with gas turbines or in internal combustion engines. The gas turbines fed with hot combustion gases are prone to rapid deterioration because of particulate and corrosive particles contained in the combustion gases. Thus, it is necessary to filter and otherwise condition the hot combustion gases before their entry into the power turbines.
In a preliminary treatment, hot gases exiting a combustion chamber are generally submitted to a cyclonic treatment to remove particulate material larger than about 10 xcexcm. Smaller particles remain in the gas stream and must also be removed to avoid corrosion, pitting and other deterioration of the turbines. These smaller particles are the target of specialized hot gas filters.
A variety of hot gas filters are known. One branch of prior art is concerned with fixed bed filters. In a fixed bed filter, hot gases pass through a mass of filtering granules. These filters require frequent cleaning and regeneration of the filtering media as the filters quickly become plugged.
Another branch of prior art is concerned with mobile bed filtering equipment where the filtering granules are constantly moved. Movement is generally achieved by pneumatic recirculation and gravitational cascade between baffles. This technique allows better interception of small particulate matter and allows filtering granule regeneration without interrupting the filtering operation. For example, U.S. Pat. No. 4,017,278 proposes the use of two concentric vertical cylindrical parts. The inner wall being perforated to permit only the passage of the gas while the outer wall is appropriately louvered to allow particles passing through with the gas while maintaining granular filtering material trapped in the annular space and in continuous movement downwards. The granules leaving the filter from the bottom of the filter being recycled back pneumatically after being cleaned in a cyclonic collector. However, the recirculation of filtration particles requires additional energy and instrumentation.
Yet another branch of prior art is concerned with fluid-bed filters. In fluid-bed filters, the filtering material is kept fluidized by the flow of hot gas. For example, U.S. Pat. No. 4,157,959 proposes a three-stage filtration method. In a first stage a cylindrical fixed bed of granules is used to filter out particles from an upward flowing gas stream; during this operation a particles layer (cake) is built at the bottom of the fixed bed, which is kept immobile by means of an adjustable rod/plate combination. In a second stage, the plate is raised, allowing the particle bed to be converted to a bubbling fluidized one. The filtration continues up to maximum allowable pressure drop occurring when the filter requires cleaning. In the last and third step, the filtering granules are regenerated. The main drawback of such technique is the requirement for moving parts and the downtime associated with filtering granule regeneration.
Thus, despite advances in the art, there remains an important need for an improved hot gas filtration system capable of overcoming the drawbacks of the prior art.
One object of the present invention is to provide a hot gas filtration apparatus which combines the benefits of a mobile granular filtration bed and a fluidized bed.
A further object is to provide a hot gas filtration apparatus capable of operating at elevated temperatures and preferably without moving parts and without the requirement for recirculating pumps.
The present invention provides a granular filtration apparatus for the removal of particulate material from hot gas by contacting said hot gas with granular filtration material. The apparatus being of the type having a filtration chamber containing granular filtration material and having a tapered lower portion comprising at least one inlet tubing for feeding therein hot gases to be filtered and at least one outlet tubing for drawing filtered hot gas. The main features of the present invention consist of a filtration unit mounted in the filtration chamber above each inlet tubing. The filtration unit comprising a first tubular member mounted above the inlet tubing so as to prevent granular material from entering or blocking said inlet tubing and a second tubular member having a bottom and a top end and being mounted essentially in a co-axial and spaced relationship above the first tubular member. During operation, the granular filtration material is siphoned up and fluidized by the hot gases as they travel up the second tubular member, the granular filtration material enters the bottom end of the second tubular member through the space between the first and second tubular member, exits at the top end of the second tubular member and falls to the tapered lower portion of the filtration chamber to be eventually recirculated and fluidized by the hot gases.
The invention also provides a novel gas inlet distribution plate fastened between the bottom tapered portion of the filtration chamber. The distribution plate is provided with a plurality of gas distribution tuyeres and is encased with the first tubular member as described above.
Other features and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that this detailed description, while indicating preferred embodiments of the invention, is given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art.